Properly weldable,corrosion resistant structural steel of high yield point

ABSTRACT

THIS INVENTION IS FOR CORROSION RESISTANT STRUCTURAL STEEL WHICH HAS HIGH TENSILE STRENGTH, A HIGH YIELD POINT AND WELDABLE UNDER ANY CONDITIONS. THE STEEL CONTAINS MANGANESE, NICKEL AND SILICON IN ADDITION TO THE GENERAL CONTAMINATES AND ALLOYING ELEMENTS AND NIOBIUM IN AMOUNTS THAT COPPER, ALUMINUM, NITROGEN AND NIOBIUM IN AMOUNTS THAT IMPART THE STATED PROPERITES THERETO.

Int. Cl. C22c 37/00, 39/02 US. Cl. 75-124 1 Claim ABSTRACT OF THEDISCLOSURE This invention is for corrosion resistant structural steelwhich has high tensile strength, a high yield point and weldable underany conditions. The steel contains manganese, nickel and silicon inaddition to the general contaminates and alloying elements andparticularly carbon, copper, aluminum, nitrogen and niobium in amountsthat impart the stated properties thereto.

The invention relates to a properly weldable corrosion resistantstructural steel of high yield point, especially for constructions,machines and buildings subject to the vicissitudes of weather.

The realization of up-to-date economic investmentsfor instancelight-structure buildings-necessitates the production and use of steelsbeing weldable under any conditions by an optional welding process, withusual working plan, that is unconditionally, providing simultaneouslyfor a corrosion resistance for several decades.

Also at present, corrosion resistant structural steels properly weldableunder suitable circumstances and having good mechanical properties areknown. Such steels are for instance the Cor-Ten-type steels developedand generally used in the United States.

They contain 0.10 to 0.19 percent C, 0.20 to 1.25 percent Mn, 0.25 to0.55 percent Cu, 0.50 to 1.25 percent Cr and about 0.10 percent V, insome cases 0.07 to 0.15 percent P and about 0.50 percent Ni (underpercent weight percent shall be understood). Also the Japanese SteelFuji Corten and the home type Korell belong to this type of steel. Therate of corrosion of these steels expressed by the average penetrationdepth of the corrosionmakes out 0.025 to 0.054 mm. per year inindustrial atomsphere. With the carbon steel tested for comparisonpurposes, the average penetration depth is 0.11 to 0.13 mm. undersimilar conditions. As for their mechanical properties, it ischaracteristic that their tensile strength (a amounts to 40 to 60 kp.per sq. mm., their yield point (11 to 30 to 50 kp. per sq. mm., theirelongation (6 to 15 to 20 percent and their impact value to 8 to 12 mkp.per sq. cm. at +20 C. and to 2.8 to 9.0 mkp. sq. cm. at 40 C.

These steels are properly weldable, due to the risk of pentration in theheat elfect zone of the welding, however, they can be welded only undersuitable conditionsfor instance preheating, subsequent heat treatment-orby special welding process. Otherwise the steel would be brittled in theheat effect zone, rendering unreliable the welded joints and limiting toa considerable extent the applicability of these steels in certainfields. A further limitation arises in case of welded constructions fromthe fact, that when welding chromium-containing steels to steels notcontaining chromium, an electrochemical corrosion occurs.

Structural steels are known the mechanical properties of which,especially with respect to the impact value stipulated as the criterionof weldability, are better than United States Patent 0 ice those of theformers. For instance the tensile strength of structural steel alloyedalso with Cu, Nb and Al (0' is 66.3 to 71.2 kp. per sq. mm., its yieldpoint (Up) 49.2 to 64.7 kp. per. sq. mm., its elongation (0' 25 to 35percent, its impact value (A,,) 10.4 to 23.1 mkp. per sq. cm. at +20 C.and 2.0 to 11.5 mkp. per sq. cm. at 45 C., in hot-rolled condition.

There are structural steels properly weldable under suitable conditionsand having high yield point, which contain in addition to the aboveenumerated alloying elements also V, Cr and Mo, these steels being,however, not corrosion resistant.

One group of structural steels known today has also a relatively goodcorrosion resistance, while due to their mechanical properties as wellas to the problems mentioned in connection with their weldability theirapplicability is limited. The other group of structural steels whilethey have relatively good mechanical properties and under suitableconditions they are properly weldablehave no satisfactory corrosionresistance.

The aim of the present invention is the production of a corrosionresistant steel having a higher yield point than the known steels, andbeing properly weldable under any conditions, which could be used asbasic material for constructions, machines and buildings as well asvehicles subject to the vicissitudes of weather.

The aim set by the invention is attained so that the steel according tothe invention contains 0.155 to 0.23 weight percent of C, 0.81 to 2.50weight percent of Cu, 0.11 to 0.20 weight percent of Al, 0.010 to 0.040weight percent of N and 0.05 to 0.20 weight percent of Nb. Some of thesealloying elements are not soluble in solid iron if being in a quantitycorresponding to the composition ratio according to the invention, theydo not form chemical compounds therewith, moreover, they reduce thedissolving of elements otherwise soluble in iron. Consequently, a denseprotective layer hardly soluble under the efiect of moisture is formedon the surface of the steel under the effect of the atmosphere, whichprotects the steel against the further corrosion caused by theatmosphere and completely excludes the occurrence of pitting.

The composition of the steel according to the invention results inexcellent mechanical properties and unconditional weldability andprovides for the corrosion resistance. These properties develop as aconsequence of the grain refining occurring under the effect ofnitrides, of the precipitation hardening of Cu and Nb, as well as of theinterstitial building-in of NbCN into the crystal lattice of iron andinto the dislocations.

The corrosion rate of the structural steel according to the inventionis-under industrial atmospheric conditions-ten times lower than that ofthe conventional steels and twice lower than that of the above describedcorrosion resistant structural steels. The corrosion product developedon the steel surface under the effect of the moisture is much lesssolubleas against the known corrosion resistant steelsbut the protectinglayer developed in this way provides for an increased corrosionresistance. Since this protecting layerdoes not cause staining, thesteel can be used as outer decorative element without painting or anyother surface treatment. Moreover, the paint tolerance capacity of thesteel according to the invention is also excellent.

The steel according to the invention is unconditionally weldable. Themeasuring number of the carbon equivalent usual for characterizing theweldability is lower than 0.55. The metallurgical structure of the steelensures that no embrittlement and coarse structure change occur in therange of the welding heat eifect.

The steel according to the invention has outstanding mechanical propertyparameters, as well as advantageous weldability and corrosionproperties, as a consequence of which a considerable reduction of weightcan be achieved with constructions, machines and buildings.

The steel according to the invention and its mechanical, weldability andcorrosion properties will be described more detailed by way of examples.

EXAMPLE 1 In a -ton arc furnace a material of the following compositionhas been produced:

Weight percent C 0.20 Mn I 1.33

Cr 0.12 Cu i 1.00 Nb 0.08 N 0.020 A1 0.13 Pb 0.22

This material had the mechanical, weldability and corrosion propertiesshown in the first column of the enclosed table when having beenhot-rolled to 22 mm., aircooled, without heat treatment and those shownin the second column of the said table in case of cooling accelerated bywater spray.

EXAMPLE 2 The material of the following composition has been producedsimilarly in a 10-ton arc furnace:

Weight percent This material had the mechanical, weldability andcorrosion properties shown in the third column of the enclosed tablewhen having been hot-rolled similarly to dia. 21 mm., then air-cooled,without heat-treatment, and

4 those shown in the fourth column in case of a cooling accelerated bywater spray.

The properties of the materials introduced in the examples are shown inTable 1.

Thus, it is to be seen that the structural steel according to theinvention has not only better corrosion properties than the corrosionresistant structural steels known today and not only unconditionalweldability but also excellent tensile properties. It should be notedthatas against to the Cr-containing Corten-type steels, for instance-thestructural steel according to the invention can be safety produced alsoin open-hearth furnace and in converter, being thus suitable formass-production.

TABLE 1 Material according to- Property Unit Example 1 Example 2 Tensilestrength Kp. per sq. mm..- 84.5 150 97.0 170 Yield point Kp. per sq.mm... 69.2 82.0 Elongation Percent 23 12 22 11 Reduction in area. do 6762 Impact value at 40 C. Mkp. per sq. cm- 18 16 12 13 Cequivalent/according 0.46 0.46 0.49 0.49

to ISO TC 17/SCI. Hiardrzriass or basic ma- HVm 222 240 err Hardness ofthe heat HV 225 273 effect zone of welding joint. Corrosion penetrationMm 0.017 0.015

depth per year in industrial atmosphere.

What we claim is:

1. Corrosion resistant structural steel of high tensile strength andyield point, properly weldable under any conditions, consistingessentially of by weight percentage, in addition to the generalcontaminating and alloying elements 1.0 to 2.50 percent of Mn, 0.30 to1.90 percent of Ni, 0.02 to 0.90 percent of Si, 0.155 to 0.23 percent ofC, 0.81 to 2.50 percent of Cu, 0.11 to 0.20 percent of Al, 0.010 to0.040 percent of N 0.05 to 0.20 percent of Nb and the balance beingiron.

References Cited UNITED STATES PATENTS 2,679,454 5/1954 Ofienhauer 751253,303,061 2/1'967 Wilson 75124 3,692,514 9/1972 Hydrean 75-124 HYLANDBIZOT, Primary Examiner US. Cl. X.R. 75-125

